Staphylococci are Gram-positive spherical cells, usually arranged in grape-like irregular clusters. Some are members of the normal flora of the skin and mucous membranes of humans, others cause suppuration, abscess formation, a variety of pyogenic infections, and even fatal septicemia. Pathogenic staphylococci often hemolyze blood, coagulate plasma, and produce a variety of extracellular enzymes and toxins. The most common type of food poisoning is caused by a heat-stable staphylococcal enterotoxin. The genus Staphylococcus has at least 30 species. The three main species of clinical importance are Staphylococcus aureus, Staphylococcus epidermidis, and Staphylococcus saprophyticus. Staphylococcus aureus is coagulase-positive, which differentiates it from the other species. S. aureus is a major pathogen for humans. Almost every person has some type of S. aureus infection during a lifetime, ranging in severity from food poisoning or minor skin infections to severe life-threatening infections.
The coagulase-negative staphylococci are normal human flora which sometimes cause infection, often associated with implanted devices, especially in very young, old and immunocompromised patients. Approximately 75% of the infections caused by coagulase-negative staphylococci are due to S. epidermidis. Infections due to Staphylococcus warneri, Staphylococcus hominis, and other species are less common. S. saprophyticus is a relatively common cause of urinary tract infections in young women. The staphylococci produce catalase, which differentiates them from the streptococci.
Both Staphylococcus aureus and Staphylococcus epidermidis have a characteristic propensity for invading skin and adjacent tissues at the site of prosthetic medical devices, including intravascular catheters, cerebrospinal fluid shunts, hemodialysis shunts, vascular grafts, and extended wear contact lenses. Within 48 to 72 hours, relatively large numbers of staphylococci are demonstrable at the site of insertion of these foreign bodies. (Archer, G. L., in Remington, J. S., et al., Current Clinical Topics in Infectious Diseases, McGraw-Hill, NY, 25-46, 1986.)
Staphylococcus epidermidis is a generally avirulent commensal organism of the human skin, and is the principal etiologic agent of infections of peripheral and central venous catheters, prosthetic heart valves, artificial joints, and other prosthetic devices. It has been demonstrated that S. epidermidis cells attach and proliferate on the inner or outer surfaces of catheters, irrespective of their composition—whether polyethylene, polyvinylchloride, polyvinylfluoride or polyester based.
Initial localized infections of indwelling medical devices can lead to more serious invasive infections such as septicemia, osteomyelitis, and endocarditis. Vascular catheters are thought to become infected when microorganisms gain access to the device, and hence the bloodstream, by migration from the skin surface down the transcutaneous portion of the catheter. In infections associated with medical devices, plastic and metal surfaces become coated with host plasma and matrix proteins such as fibrinogen, vitronectin and fibronectin shortly after implantation. S. epidermidis bacteremia can result in an excess hospital stay of 8 days, which is quite expensive.
Although the virulence of coagulase-negative staphylococci is enhanced in the presence of a foreign body, the microbial factors that permit these normal skin commensals to become nosocomial pathogens have not been well characterized. The ability of coagulase-negative S. epidermidis to adhere to these proteins is of crucial importance for initiating infection. As adherence is believed to be the critical first step in the pathogenesis of coagulase-negative staphylococcal foreign-body infections, attention has focused on surface properties of these organisms that might mediate adherence to, and then colonization of, polymeric prosthetic materials.
A number of factors influence an organism's ability to adhere to prosthetic material. These include characteristics of the microorganism and the biomaterial, and the nature of the surrounding environment. The initial attraction between the organism and the host is influenced by nonspecific forces such as surface charge, polarity, Van der Waal forces and hydrophobic interactions. The critical stage of adherence involves specific interactions between cell surface adhesins and immobilized host proteins. To date, investigation concerning the adherence of S. epidermidis to biomaterials has concerned itself primarily with the role of the extracellular polysaccharide or glycocalyx, also known as slime. Despite intensive study, however, the proposed role of slime in the pathogenesis of disease or even its composition remain debated. (Drewry et al., Clin. Microbiol 28:1292-1296, 1990) Currently, extracellular slime is thought to play a role in the later stages of adherence and persistence of infection. It may serve as an ion exchange resin to optimize a local nutritional environment, prevent penetration of antibiotics into the macro-colony or protect bacteria from phagocytic host defense cells. Peters et al. have shown by electron microscopy studies that extracellular polysaccharide appears in the later stages of attachment and is not present during the initial phase of adherence. (J. Infect. Dis., 65146:479-482, 1982) Hogt et al. demonstrated that removal of the extracellular slime layer by repeated washing does not diminish the ability of S. epidermidis to adhere to biomaterials. (J. Gen. Microbiol. 129:2959-2968, 1983)
Thus far, study of exopolysaccharide has lent little to prevention of initial adherence by the bacteria. Several other studies have identified other potential adhesins of S. epidermidis including the polysaccharide adhesin (PS/A) observed by Tojo et al. (J. Infect. Dis. 157:713-722, 1988) and the slime associated antigen (SAA) of Christensen et al. (Infect Immun, 58:2906-2911, 1990).
It has been demonstrated that PS/A is a complex mixture of monosaccharide adhesins which blocks adherence of PS/A producing strains of S. epidermidis. In an animal model of endocarditis antibodies directed against PS/A were protective. However, it is not clear whether this protective effect was specific, related to anti-adhesive effects of the antibody or due to a more generalized increase in the efficiency of opsonophagocytosis of blood borne bacteria. It has been hypothesized that each adhesin functions in different stages of the adherence process with one or more of these adhesins responsible for initial attraction while others are needed for aggregation in the macro-colonies.
Despite many studies, factors involved in the initial adherence of S. epidermidis to biomaterials remain largely unknown. Further unknown is a practical method for preventing the first stage of infection, adherence or adhesion. Therefore, a great need remains for the discovery and characterization of bacterial adhesin proteins and the genes that encode them.
Accordingly, it is an object of the present invention to provide cell-wall associated extracellular matrix binding proteins of coagulase-negative staphylococci.
It is a further object of the present invention to provide coagulase-negative staphylococcal surface proteins that are able to inhibit staphylococcal adhesion to the immobilized extracellular matrix or host cells present on the surface of implanted biomaterials.
It is a further object of the present invention to provide a coagulase-negative staphylococci vaccine, to generate antisera and antibodies to coagulase-negative staphylococcal proteins, and to isolate antibodies to coagulase-negative staphylococci.
It is a further object of the present invention to provide improved materials and methods for detecting and differentiating coagulase-negative staphylococcal organisms in clinical and laboratory settings.
It is a further object of the invention to provide nucleic acid probes and primers specific for coagulase-negative staphylococci.
It is a further object of the invention to provide methods for detecting, diagnosing, treating or monitoring the progress of therapy for bacterial infections that are sensitive and specific for coagulase-negative staphylococci.
These and other objects, features and advantages of the present invention will become apparent after a review of the following detailed description of the disclosed embodiments and the appended claims.